Hello Christian,
I've moved your topic here, since if I understand correctly you're referring to creating a virtual piston within an organ definition (ODF) that you've created yourself.
Note that you could instead simply assign the desired function to one of Hauptwerk's piston toolbar pistons (View | Piston toolbars ...') by right-clicking on the desired piston. Also, you could access that function via the relevant control panels, and/or assign it to be triggered from MIDI by auto-detecting in the usual way.
However, assuming that you do specifically want to embed a virtual piston/switch in your ODF for the purpose for some reason, and briefly (since we only officially provide support for editing ODFs to major sample set producers):
To trigger the "Registration: comb. set: favorite 01" function, the code that you would need to specify for Switch.DefaultInputOutputSwitchAsgnCode is 901328 (not 910341) -- see the organ definition format documentation (which you have access to via the 'Design tools' menu in Hauptwerk).
Note also that (regardless of how you trigger it) that function will load the assigned combination set if the setter is off, and assign the combination set if the setter is on.